The ability to accurately sense infrared radiation has become an important technical consideration in a variety of military and civilian applications. The sensors used to detect the infrared radiation must be protected from environmental hazards. This is usually done by placing the sensors behind infrared transmissive windows or domes. During the lifetime of the electro-optic system, these protective windows and domes suffer from environmental degradation. This is particularly true of windows and domes designed to protect sensors receiving the eight to twelve micron wavelength region of infrared radiation as these protective windows and domes are commonly constructed from zinc sulfide (ZnS), germanium (Ge), zinc selenide (ZnSe) and gallium arsenide (GaAs). The environmental degradation of the protective windows and domes is the result of exposure to rain and other particle wear which can cause impact damage and general erosion of the outer surface of the protective window or dome. This degradation results in a loss of transmission through the component and at times can result in catastrophic failure of the dome resulting in the destruction of the protected electro-optic sensor. The environmental degradation discussed previously is exacerbated if the window or dome is on a high speed platform like a plane or a missile as is common in military applications.
For the most part, efforts to construct protective windows and domes having more resistance to environmental degradation have focused on developing protective coatings for the windows and domes. This is due to the limited choice of materials for the domes and windows themselves which allow transmission of infrared radiation in the eight to twelve micron wavelength region. Coatings comprising diamond-like carbon have been developed and used in the past. Diamond-like carbon offers some wear resistance but can only be used as very thin coatings due to the optical absorption of the material. For example, a typical diamond-like carbon coating can only be one to two microns in thickness without appreciably affecting the optical characteristics of the dome or window. Other materials such as boron phosphide and germanium carbide are being developed but still have shown unacceptable levels of optical absorption at coating thicknesses sufficient to provide impact and wear resistance for the dome or window.
Accordingly, a need has arisen for an improved infrared transmissive window to protect electro-optic components.